Today's three panel projection systems have the drawback of bulky beam splitter units. The white light is first split by many dichroic mirrors, folding mirrors or polarizing beam-splitters into the primary colors red, green and blue and after being reflected or transmitted by the panels, the light again is recombined by several prism cubes before it is projected by a common projection lens. In the case of reflective panels, this results in a long back-focal length BFL (=distance between the panels and the first surface of the projection lens). A long BFL complicates the design of the projection lens. FIGS. 1a and 1b show two commonly used architectures of optical engines with three reflective panels. In FIG. 1a, the so called “3PBS architecture”, light is split by dichroic mirrors and folding mirrors and is recombined by an X-cube. In FIG. 1b, the so-called “ColorQuad” architecture, the light is split and recombined by four polarizing beam-splitters together with wavelength-selective retarders.
A FIG. 2A shows a basic architecture according to the present invention. It consists of two polarizing beam-splitters PBS1 and PBS2, one wavelength selective optical element WSOE, three display panels P1, P2, P3 and two projection lenses PL1 and PL2 building the optical projection unit OP. The WSOE is placed between the polarizing beam splitters PBS1 and PBS2. One panel is attached to PBS1 and two panels are attached to PBS2. Projection lens PL1 is attached to PBS1 and PL2 is attached to PBS2.
White and s-polarized primary illumination light L1, w is entering the polarizing beam-splitter PBS1 and is reflected by the polarizing beam-splitter coating in direction of the wavelength selective optical element WSOE. One spectral part SP1 of the white light beam L1, w is reflected back by the WSOE thereby changing its polarization state from s- to p-polarization. The p-polarized spectral part SP1 is now transmitting the PBS1 and is entering the display panel P1. The spectral parts SP2 and SP3, which are completely or partly distinct to each other and to SP1, are transmitting the WSOE, thereby changing the polarization state of the spectral part SP2 from s- to p-polarization. The transmitted and still s-polarized spectral part SP3 is reflected by the beam splitter coating of PBS2 and is entering the display panel P3. The p-polarized spectral part SP2 is passing the beam splitter coating of PBS2 and is entering the display panel P2.
In the ON state the polarization states of the spectral parts SP1 and SP2 are changed from p- to s-polarization and the polarization state of the spectral part SP3 is changed from s- to p-polarization after being reflected by the display panels P1, P2 and P3 respectively: see FIG. 2B. Now the s-polarized spectral part SP1 is reflected by the beam-splitter coating of PBS1 and is entering projection lens PL1. The s-polarized spectral part SP2 is reflected by the beam-splitter coating of PBS2 and is entering projection lens PL2. The p-polarized spectral part SP3 is passing PBS2 and is entering projection lens PL2.
In the OFF state the polarization state of the spectral parts SP1 and SP2 remains p-polarized and the polarization state of the spectral part SP3 remains s-polarized after being reflected by the display panels P1, P2 and P3 respectively. All spectral parts SP1, SP2 and SP3 are now redirected in direction of the illumination unit: see FIG. 2C.
B A common illumination optical unit may be attached to PBS1, as shown in FIG. 3. White light L1, w is emerging from this illumination unit and is entering PBS1. The illumination optical unit is adapted to focus the illuminating light beam to the panels P1, P2, P3. The wavelength selective optical element WSOE is placed between PBS1 and PBS2 in such a way that the optical distance of spectral part SP1 to panel P1 is same like the distance of spectral parts SP2 and SP3 to the panels P2 and P3 respective.
C The wavelength selective optical element WSOE comprises two quarter wave layers Q1 and Q2, a dichroic layer D on a transparent substrate S, e.g. glass and a wavelength dependent retarder R as shown in FIG. 4.
Linear s polarized white light entering the WSOE is passing the first quarter-wave layer, thereby turning the polarization state from linear to circular. Next the light beam hits the dichroic layer D, thereby reflecting spectral part SP1 and changing the chirality of the circular polarized light, e.g. from left circular to right circular. The reflected back spectral part SP1 next passes again the first quarter-wave layer, thereby turning the polarization state to linear p.
The transmitted spectral parts SP2 and SP3 are next passing the second quarter wave layer Q2, thereby turning the polarization state from circular to linear. By passing the wavelength selective retarder R the polarization state of spectral part SP2 is turned from s-polarized to p-polarized or—alternatively the polarization state of spectral part SP3 is turned from p-polarized to s-polarized.
D Light of spectral part SP2 is reflected in p-polarized mode from the display panel P2 when the panel is in the OFF state as shown in FIG. 5. Ideally, all of the p-polarized light is passing the PBS2 in direction to PBS1. But, as a general attribute of polarizing beam-splitters, about 10% of the p-polarized light is reflected at the polarizing beam-splitter coating. Therefore—even in the OFF (=black) state—about 10% of the light would enter the projection lens, resulting in a worse contrast.
To overcome this loss in contrast, an additional clean-up polarizer in front of the projection lens PL2 is required to block the leaking p-polarized light. This clean-up has to be a wavelength selective polarizer WSP, because only p-polarized light of spectral part SP2 must be blocked. The p-polarized light of spectral part SP3, which comes from display panel P3 in the ON state, must be transmitted by the WSP.
E The wavelength selective polarizer WSP mentioned in section D can be of following types:                Cholesteric polarizer (CF) with adapted quarter-wave layers. This CF reflects the leaking p-polarized light of spectral part SP2, but lets through spectral part SP3 and all s-polarized light of spectral part SP2.        Colour selective retarder, which turns the polarization state of spectral part SP2 from p polarized to s polarized and from s polarized to p polarized and leaves the polarization state of spectral part SP3 as it is. An additional absorbing polarizer then absorbs all s-polarized light.        An absorptive wavelength selective polarizer which is adapted in such way that p-polarized light of spectral part SP2 is absorbed, but p-polarized light of spectral part SP3 and s-polarized light from SP2 is transmitted.        
F Light of spectral part SP1 is reflected in p-polarized mode from display panel P1 when the panel is in the OFF state as shown in FIG. 5. Ideally, all of the p-polarized light is passing the PBS1 in direction to PBS2. But, as a general attribute of polarizing beam splitters, about 10% of the p-polarized light is reflected at the polarizing beam splitter coating. Therefore—even in the OFF (=black) state—about 10% of the light would enter the projection lens, resulting in a worse contrast.
To overcome this loss in contrast, an additional clean-up polarizer or analyser A in front of the projection lens PL1 is required to block the leaking p-polarized light. In contrast to section D this clean-up polarizer A can be of standard type, as only light in the spectral part SP1 is influenced.
G In reality the diameter of the projection lenses PL1 and PL2 exceed the diameter of the polarizing beam-splitter cubes PBS1 and PBS2. As a result, the distance between PBS1 and PBS2 must be large enough to fit to both projection lenses PL1 and PL2. Especially rear projection lenses have front lenses with a large diameter.
To overcome this space requirement the projection lenses could be split in two separate first lens blocks LB1 and LB2 attached to PBS1 and PBS2, respectively, and a common second or front lens block FLB as shown in FIG. 6A. Folding mirrors FM1, FM2 combine the two separate light paths coming from the first lens blocks LB1 and LB2 into the common front lens block FLB. An X prism X combines the light coming from lens block LB1 with light coming from lens block LB2. The X prism X has two different dichroic coatings. One coating is to reflect the light of spectral part SP1 and the other to reflect the light of spectral parts SP2 and SP3 into the common front lens block FLB.
H Alternatively, the X prism X can be rotated by 90° in order to fold the common light path together with the front lens block FLB out of the plane as shown in FIG. 6B. This has the advantage of small footprint and adapts the engine to be used in rear projection cubes.
I Instead of using an X prism X to recombine the two light paths into one an arrangement according to FIG. 7 can be used. It comprises three folding mirrors FM1, FM2, FM3 and one dichroic mirror prism DM to recombine the two light paths.
Existing projection systems with three reflective display panels are using three to four beam splitter cubes and need a long back-focal length of the projection lens. The invention describes a projection system with outstanding compact illumination and beam splitter part using only two beam-splitter cubes. The projection lens or lenses has/have a short back focal length.